1. Field of the Invention
This invention relates to a method of making crystallized glass, and in particular, to a method of making such glass when starting with lithium carbonate or the like, rather than the mineral petalite, as the source of lithium values. In one aspect, the invention concerns avoiding microcracking of the glass surface during the heat treatment used to crystallize the glass by the purposeful addition of potassium, rubidium or cesium values.
2. Description of the Prior Art
Glass-ceramic articles produced by devitrification or crystallization of glass are well known in the art. In U.S. Pat. No. 3,625,718 to Petticrew, there are disclosed glass compositions which are particularly suited for crystallization. Such compositions comprise on a percentage by weight basis from 64 to 74 percent SiO.sub.2, from 15 to 23 percent Al.sub.2 O.sub.3, from 3.3 to 4.8 percent Li.sub.2 O, from 1 to 3.8 percent ZnO, from 1.2 to 2.4 percent TiO.sub.2, from 0 to 2 percent ZrO.sub.2, and wherein the named ingredients constitute at least 90 percent by weight of the total glass composition. Additional components such as As.sub.2 O.sub.3 and Sb.sub.2 O.sub.5 sometimes end up in the final glass composition because they are added to the glass batch to act as fining agents.
Such glass compositions, as disclosed above, can be obtained by suitably melting a mixture of sand, petalite, hydrated alumina, mixed zinc and zirconium silicates, arsenic trioxide, antimony trioxide, zirconium oxide, titanium dioxide and a suitable quantity of appropriate cullet. After the batch has been suitably melted, fined, homogenized and pressed into plates, the composition can be heat treated to crystallize or devitrify the glass. At the end of the heat treating period, the crystalline phase constitutes about 98 percent of the composition with the remainder being a glassy phase.
Crystallized glass articles offer significant advantages over ordinary amorphous glass articles in that crystallized glass has a high modulus of rupture and a very low linear coefficient of thermal expansion, generally below 20 .times. 10.sup.-.sup.7 per .degree.C., which makes these glasses useful as cooking surfaces for kitchen ranges, hot plates and the like.
As has been mentioned, it has been customary to use the mineral petalite as the source of lithium values in making the glass of the kind indicated above. Petalite consists principally of lithium aluminum silicates, a typical analysis being 4.29 percent Li.sub.2 0, 16.15 percent Al.sub.2 O.sub.3, 77.1 percent SiO.sub.2, 0.49 percent Na.sub.2 O, 0.31 percent K.sub.2 O and 0.026 percent Fe.sub.2 O.sub.3. Petalite is in very short supply, being available substantially only from Rhodesia. When efforts were made to produce crystallized glass of the kind indicated above, but with the use of lithium carbonate and the like in place of the petalite, microcracks developed in the glass surface during the heat treatment used to crystallize the glass. The resultant glass would generally be weaker than normal crystallized glass, and because of the tendency of the microcracks to catch dirt, the cracked crystallized glass sheets or plates stained easily and were difficult to clean, making them commercially unacceptable. Unfortunately, the prior art did not afford any clear indication of what to do to overcome the problem.
It is known in the prior art that sources other than petalite can be used in the glass batch as a source of lithium values, for example, U.S. Pat. Nos. 3,489,577 to Tashiro et al., 3,352,656 and 3,352,698 and British Pat. No. 1,028,871, all to McMillan et al., and U.S. Pat. No. 3,006,775 to Chen, disclose the use of lithium carbonate in the glass batch. These same references also disclose the optional incorporation of a potassium salt into the glass batch. Surprisingly, there is no disclosure in these references of a microcracking problem or of the effect that potassium has in eliminating the problem.
Therefore, it appears that the prior art has not recognized the problem of glass surface microcracking which occurs in certain crystallized glass compositions during the heat treatment. This invention has uncovered the problem and provided a solution.